Infrared spectroscopic evidence of conformational transitions of an atrial natriuretic peptide.

Abstract

The conformational properties of the atrial natriuretic peptide atriopeptin III were investigated by Fourier-transform infrared spectroscopy. Infrared spectra in the amide I region were analyzed quantitatively using deconvolution and band-fitting procedures. According to this analysis, in aqueous solution the monomeric peptide has a random structure. Binding to bilayer vesicles of dimyristoyl phosphatidylglycerol results in drastic conformational changes. The lipid-complexed atriopeptin III adopts a highly ordered structure of predominantly fl-sheets. A transition to a similar, but not identical, f-structure occurs upon self-association of the peptide. The results of model experiments suggest that the binding of this atrial peptide to the target cell membrane is associated with the induction of fl-sheet structure and that it is this latter conformation that is predominant in the active form of the hormone. Ser-Ser-Cys-Phe-Gly-Gly-Arg-Ile-Asp-Arg-Ile-GlyAla-Gln-Ser-Gly-Leu-Gly-Cys-Asn-Ser-Phe.Arg-Tyr The molecular dynamic basis for the physiological action of the atrial peptides and, in particular, the conformational properties of the peptides are, however, largely unknown. In the present communication we have used infrared spectroscopy to establish the conformation of atriopeptin III (molar ratio 10:1) in buffer, followed by rapid dispersion on a Vortex mixer. Alternatively, samples were prepared by rapidly mixing a freshly prepared peptide solution (2 mM) with a suspension of sonicated lipid vesicles. The infrared spectra of samples prepared by the two procedures were identical. Infrared spectra were recorded at 30'C with a Digilab FTS-15 instrument using a high-sensitivity mercury cadmium telluride detector. For each spectrum, 512 interferograms were added and Fourier-transformed to give a resolution of 2 cm-1. The spectra in the 1500 to 1800 cm-' region were corrected for the weak underlying 2H20 absorption. To eliminate spectral contributions of atmospheric water vapor, the instrument was continuously purged with dry nitrogen. Overlapping infrared bands were resolved using Fourier self-deconvolution procedures (4,5). Curve fitting was performed as described by Fraser and Suzuki (6). Fig. 1A shows an infrared spectrum, between 1500 and 1800 cm-1, of a freshly prepared 4 mM atriopeptin III solution in 2H20 buffer (recorded within 10 min after sample preparation). The conformation-sensitive amide I mode between 1600 and 1700 cm-' is dominated by a broad, featureless band with a maximum at 41645 cm-'. Whereas the freshly prepared solution of atriopeptin III was optically clear, prolonged incubation of the sample (at 30'C) resulted in an increase in turbidity, suggesting a time-dependent aggregation of the monomeric peptide molecules. This process was accompanied by drastic changes in the infrared spectrum. Upon aggregation of atriopeptin III, the maximum of the amide I band was shifted by =20 cm-1 to lower frequency (Fig. 1B). The process of self-association was dependent on the peptide concentration. ...

Impact of Strong and Weak Lipid-Protein Interactions on the Structure of a Lipid Bilayer on a Gold Electrode Surface

“…[19,24,[27][28][29][30][31] Changes in the secondary structure of a protein binding or incorporating onto lipid membranes, vesicles or dispersions by means of IRS have been widely discussed in the literature. [19,27,30,31] Much less is known about the impact of lipid-protein interactions on the structure of lipid assemblies. [19,28,29] Thermotropic IRS studies on the interaction of gramicidin A, alamethicin and bacteriorhodopsin with lipid liposomes showed an increase in the number of gauche conformers and increased mobility of the hydrocarbon chains in lipid vesicles existing in the liquid-crystalline phase.…”

“…[26,[86][87][88] Bands centred at 1691, 1676 and 1627 cm À1 are due to b sheets with antiparallel oriented strands. [30,88,89] The band at 1676 cm À1 in H 2 O solution may also indicate the presence of turns and loops in the protein structure. [30,88] The position of the amide I mode at 1643 cm À1 in H 2 O indicates the presence in MAG-Fc of b sheets with parallel oriented strands.…”

“…[30,88,89] The band at 1676 cm À1 in H 2 O solution may also indicate the presence of turns and loops in the protein structure. [30,88] The position of the amide I mode at 1643 cm À1 in H 2 O indicates the presence in MAG-Fc of b sheets with parallel oriented strands. [26,89] The IR spectrum of MAG-Fc agrees with spectra of other immunoglobulins reported in the literature.…”

“…Indeed, in IR spectroscopy turns are associated with a characteristic band around 1665 cm À1 that as been reported in the spectra of many proteins. [26,30,88] The presence in the spectrum recorded in D 2 O of an absorption band at 1648 cm À1 suggests that in MAG-Fc bound to the lipid bilayer protein fragments have ill-defined, non-ordered conformation. [30,88] On the one hand, this difference may be due to the fact that the PM IRRA spectra were recorded in D 2 O whereas the solution spectrum was recorded in H 2 O.…”

“…[26,30,88] The presence in the spectrum recorded in D 2 O of an absorption band at 1648 cm À1 suggests that in MAG-Fc bound to the lipid bilayer protein fragments have ill-defined, non-ordered conformation. [30,88] On the one hand, this difference may be due to the fact that the PM IRRA spectra were recorded in D 2 O whereas the solution spectrum was recorded in H 2 O. [30,88] On the other hand, a red shift of the position of the amide I mode may indicate changes in the hydrogen-bonding network of b sheets upon binding of MAG-Fc to the bilayer.…”